1. Field of the Invention
This invention relates to a lighting system for illuminating the eye and more particularly to a lighting system for video based tracking and correcting for eye movement during vision correction treatments.
2. Description of Related Art
Pupil position data obtained by eye tracking systems is used to detect eye motion during vision correction treatments. Conventional video based eye tracking systems automatically recognize and track the position of eye positions based on landmarks present within an image of a human eye. Such equipment requires illumination of the eye by infrared (IR) light. IR light typically 850 to 930 nanometers (nm) is used because it provides a good picture contrast between the pupil and iris. Additionally, the use of IR light decouples this lighting source from other visual sources which do not contain the infrared wavelengths.
The eye, illuminated by invisible IR light, is scanned by an infrared sensitive video camera. Under normal conditions, the pupil of the eye appears as a dark hole to the illumination. The dark pupil image is input to a real-time eye tracking system consisting of a digital image processor that outputs pupil size and position coordinates relative to the scan of the camera. The eye tracking system includes a circuit and processor designed to acquire and track the dark pupil position even in the presence of shadows or other clutter normally found in images of the eye.
FIG. 1 shows a conventional eye tracking system including illumination of the eye during laser vision correction surgery. The conventional illumination system includes one or two infrared light bundles 10, mounted on a central hub 12, to illuminate the eye for tracking by the eye tracking system. The path of a visible light beam used during vision correction treatments is shown at 14. A camera 18, sensitive to IR illuminations and fixed with respect to the subject's head, scans the eye to provide a video image for tracking the position of the eye.
Conventional illumination systems such as that shown in of FIG. 1 require that light bundles 10 be relatively close to the eye in order to achieve an evenly distributed illumination. Generally, the light bundles 10 are about 80 millimeters (mm) from the eye being treated, as shown by the dimension A in FIG. 1. This narrow spacing is a significant disadvantage because the light bundles 10 may interfere with the physician's hands either prior to or during the actual vision correction treatment. Additionally, the physician's hands can inadvertently block light emitted from one or both of the light bundles 10 causing a system efficacy and/or safety problem. Under certain circumstances, the patient's brow or nose can block light emitted from the light bundles 10 causing uneven illumination of the eye.
Additionally, conventional illumination systems such as that shown in FIG. 1 require the light bundles 10 to be within 0 to +/-20 degrees to the visual axis in order to achieve generally presumption of an even illumination. If not, the light bundles 10 must be painstakingly adjusted for each patient at the time of treatment to achieve the best possible illumination based on the shape of that particular patient's face, eyes, etc. This is a time-consuming process and may result in errors.
However, even if the light bundles 10 are positioned within the conventionally preferred 0 to +/-20 degrees to the visual axis, there is an additional problem caused by specular reflections. FIGS. 2 and 3 show specular reflections 21 at an eye 17 caused by any illumination system composed of point sources, e.g. light bundles 10. Because of the relatively narrow angle (0 to +/-20 degrees) at which the light beams 19 (FIG. 3) are delivered, the specular reflections 21 can occur either within the pupil 18 or even worse at the pupil/iris border 20. This makes the machine vision computer task of defining the pupil/iris border 20 much more complex and error prone.
The system shown in FIG. 1 was originally designed for research studies of normal eyes in which the epithelium is intact with a tear layer providing distinct specular reflections. These conditions no longer hold true for current laser vision correction techniques. In particular, either the epithelium is removed under a technique called surface Photorefractive Keratectomy (PRK), or a flap is cut with a microkeratome, the flap is folded back, and the treatment is performed on the underlying stromal layer (called LASIK). Both LASIK and PRK markedly affect Illumination for eye tracking systems. The eye, rather than being shiny and smooth, becomes dryer and more diffusely reflective. This often leads to additional time-consuming physician adjustment of the light bundles 10 during a critical time period of the surgery.
Another problem exists with conventional eye tracking lighting systems when used with laser vision correction treatments. An ablating laser beam dries and roughens the eye surface further obscuring the camera's view of the pupil/iris border 20 and the peripheral limbal border. Empirically, this disadvantageous effect is accentuated by relatively coaxial lighting provided by singular light sources placed close to the eye at angles of less that +/-20 degrees.